1. Field of the Invention
This invention relates to a method for effecting the reaction of a polyhydric compound and a polyisocyanate to provide polymers comprised of urethane and isocyanurate linkages.
2. Description of the Prior Art
Rigid foams of a cured polymer comprised of urethane and isocyanurate linkages are known in the art for having excellent insulating and structural integrity properties in addition to exhibiting a high degree of fire retardancy. Polyurethane foams per se, while having excellent physical strength properties, are nonetheless sorely deficient in fire retardancy. Polyisocyanurate foams, on the other hand, are relatively non-combustible but untolerably friable for most uses. Combining the two types of polymeric structures in one polymer, however, compensates for the indicated deficiency associated with the respective structures in rigid foam applications.
While as indicated, polyurethane-polyisocyanurate foams are known, they have heretofore been comparatively difficult to prepare. The problem stems from the fact that two different reactions are involved in preparing said foam; namely, urethane formation and the trimerization of isocyanate groups to provide the isocyanurate configuration. Urethane formation is rapidly accomplished in the presence of a suitable catalyst, a host of which are known to serve effectively for this purpose. The trimerization reaction, however, is far more difficult to achieve especially in the time strictures imposed in carrying out the various foaming operations. Although the trimerization reaction can be catalytically induced, there are comparatively few catalysts applicable for this purpose and those that do promote the reaction are required to be used in combination with a conventional urethane catalyst. Accordingly, there is a need for a catalyst which will serve to effect both of the indicated types of catalysis and also exhibit substantially more effectiveness for inducing trimerization than the known catalysts hitherto available for this purpose.